Resistance spot welding of aluminum processes traditionally has a lower electrode life than gauged steel and coated steel resistance welding methods. Increased electrode life is desired because it reduces maintenance cost; increases weld quality, and most importantly yields higher production. One disadvantage of resistance welding of aluminum is erosion of the electrodes, which results in changes of the tip geometry of the electrode contact surfaces. Changes in electrode tip geometry causes irregulars in pressure and current distribution at the faying surface of the welded metal sheets throughout the life cycle of the welding process and eventually results in insufficient or discrepant welds. One example of a discrepant weld obtained during peel testing is depicted in FIG. 1.
Prior methods to increase the life cycle of resistance spot welding of aluminum modify the contact resistance of the aluminum welding surfaces either through mechanical and/or chemical means. A few examples include: twisting electrodes, arc cleaning the aluminum sheet's surface, and differential surface treatments. While these techniques can enhance performance, they are not easily incorporated into existing resistance spot welding processes and disadvantageously increase process cost.
Another method of increasing the life cycle of the electrodes in resistance spot welding is current stepping. Current stepping is the increase of current to compensate for the increase in surface area of the electrode contact surface due to erosion. Typically, current stepping is programmed by the user in increments of welds performed. For example, welds 1-100 being conducted at 24.0 kA and welds 10-200 being conducted at 25.0 kA.
Additionally, the lifecycle of resistance spot welding of aluminum metal may be increased by mechanically polishing electrodes at a frequency of about 14-20 welds between polishing. In this method, the tool and abrading medium, such as sandpaper, scotchbrite or cutting tools, are placed between the electrodes under low pressure and rotated until the intermetallic that is produced on the electrode tip during electrode erosion is removed. Similar to the above-described prior solutions to increasing the life cycle of resistance welding, mechanical polishing of electrodes is not easily incorporated into existing processes and disadvantageously increases production cost.
U.S. Patent Application Publication 2005/0045597 A1, entitled “Resistance Welding Control Method”, to Wang et al. (“Wang et al.”) describes one prior method of monitoring weld quality in resistance welding. Wang et al. disclose that weld quality may be monitored by measuring dimensional changes in the weld indentation at the welding surface. Wang et al. further discloses that weld quality may be improved by varying the welding current in response to dimensional changes in the weld indentation of the welding process. Similar to other conventional welding processes, Wang et al. do not measure dimensional changes in the electrode due to electrode erosion. Further, Wang et al. do not correlate dimensional changes in eroding electrodes to irregulars in pressure at the faying surface of metal sheets throughout the welding process.
What is needed is a method of resistance welding that increases electrode lifetime without substantially increasing production cost.